In recent years, various kinds of electric field detectors have been put into practical use. Particularly, there is widely used in such detectors a photoelectric converting element. An electric field detector of this kind can be used in various monitoring devices and the like. The assignee of the present applicants has also proposed a pressure responsive monitoring device for vacuum type electrical apparatus as a monitoring device for measuring the vacuum pressure of a vacuum circuit interrupter.
In vacuum type devices it is required to monitor the vacuum pressure in order to maintain the preferred superior dielectric characteristics of vacuum for use in power interrupting devices, as opposed to the use of special arc extinguishing materials, such as gases and liquids. Since vacuum offers a dielectric strength with a high voltage recovery rate, interruption can normally be anticipated at the first current zero in an A.C. current waveform. Further, a short contact gap can perform the interruption of current. The short gap provides a low mechanical shock.
Normally, the operating sequence of the vacuum circuit interrupter from a fault to a clear may be accomplished in less than three cycles. Since energy dumped into a fault is proportional to time, the faster cleaning action means less damage, lower contact erosion, longer maintenance free contact life, and maximum equipment protection. An important problem in the vacuum type electrical devices is that the characteristics of the devices are affected by the vacuum pressure. Namely, the problem with the use of vacuum circuit interrupters is that if there is a loss of vacuum as by leakage of air through a crack caused by undue mechanical stress, both the high dielectric strength of the vacuum dielectric and the rapid recovery rate are lost. The small contact spacing will no longer be able to sustain the high voltages. Arcs and flashovers will occur. The white arc will burn the electrode and melt the envelope, and may even extend into and attack other parts of the interrupter assembly.
In recent years, various kinds of pressure measuring systems for vacuum type electric apparatus such as the vacuum circuit interrupters have been put into practical use. As one of the pressure measuring apparatus, a pressure responsive measuring device has been proposed in copending U.S. application Ser. No. 246,617 by T. Fukushima for Pressure Responsive Monitoring Device for Vacuum Circuit Interrupters, filed Mar. 23, 1981, and in copending U.S. application Ser. No. 267,331 by Fukushima et al. for Pressure Monitoring System for a Vacuum Circuit Interrupter, filed May 26, 1981. The pressure responsive measuring device employs an electric field detecting member for detecting the change of electric field strength of the vacuum type electric device corresponding to the change of vacuum pressure.
The electric field detector of FIG. 1 comprises an electric field generating member 100 to be tested, a light source 50 for generating light, an electric field detecting member 60 for detecting electric field and for converting variation of the electric field strength to optical energy variation responsive to the electric strength, a photoelectric converting member 70 for converting optical energy to electrical energy supplied from the electric field detecting member and a discriminating circuit 80 outputting an electric signal.
The light source 50 is provided with a light emitting diode generating light in accordance with electric current flowing thereto. The electric field detecting member 60 is disposed on and/or in the vicinity of an electric field generating member. The electric field detecting member 60 is interconnected with the light source 50. The electric field detecting member 60 comprises a polarizer 62, an electric field sensitive element in the form of a Pockels cell 64 and an analyzer 66. The Pockels cell 64 is arranged between polarizer 62 and the analyzer 66. The analyzer 66 is connected to the photoelectric converting member 70. The vacuum pressure change discriminating member 80 is electrically connected to the photoelectric converting member 70, and an electrical output signal is employed as an alarm signal, an indicating signal and the like.
In the electric field detector of FIG. 1, the light produced by the light source 50 is a random polarized light 52. The random polarized light 52 is supplied to the electric field detecting member 60. In the electric field detecting member 60, the random light is polarized by the polarizer 62 to produce a linearly polarized light having a direction of polarization which is represented by an arrow. The linearly polarized light is applied to the Pockels cell 64. An electric signal in the form of electric field strength E is applied to the Pockels cell 64 from the electric signal generating member 100. The Pockels cell 64 causes the direction of polarization to change. The anaylzer 66 is provided such that its plane of polarization is perpendicular with respect to the optical axis. The electric field strength to be applied to the Pockels cell 64 is determined by the electric field strength. The light from the Pockels cell 64 is dependent upon the electric field strength E and is supplied to the analyzer 66.
In the electric field detector of the prior art, the Pockels cell is, generally, made from a material which is made of a potassium dihydrogenphosphate (KH.sub.2 PO.sub.4), so called KDP, or an ammonium dihydrogenphosphate (NH.sub.4 H.sub.2 PO.sub.4), so called ADP. Since the ADP and the KDP have large dielectric constants which are ten times that of a quartz, the electric distribution field is disturbed thereby and deviations appear in the measured electric field strength. Additionally, the ADP and the KDP are deliquence and expensive.
Quartz may be used as the Pockels cell which has the low dielectric constant. When quartz is used as the Pockels cell, correct measurement can be obtained since the electric field distribution is not so disturbed, as well as the quartz has the constant light energy loss and has no deliquence. Quartz has, however, a natural double refraction so that the polarizing plane is rotated regardless of the electric field strength, when the linearly polarized light is injected to the quartz in the optical axis direction. Furthermore, quartz is inadequate to satisfy the requirements for accuracy and reliability of the electric field detector, because the double refraction is dependent upon the thermal characteristics.
In order to avoid the defects of quartz, the Pockels cell can be made by coupling two quartz elements in series and in an air-tight configuration, each of which has the symmetrical double refraction such that the natural double refraction is eliminated. The quartz is generally cut by 45.degree.-Z cut and, therefore, the thermal characteristics of the polarizing plane is not good, as shown in FIGS. 4 and 5. In the phase Pockels cell, phase difference of the two specific oscillations of the polarizing plane is proportional to the electric field strength and to the length of element of the optical path direction. Furthermore, it is difficult to manufacture the Pockels cell, because high accuracy is required in order to adhere the two quartz elements such that an air layer is not formed therebetween and so that an optical defect does not exist.